This print is of a typical weight box or tank used by trucks in the heavy hauling load business when weight is needed over the rear drive axles. The tank is filled with water as needed for the desired weight, and emptied when not necessary, along with rings on the top to lift it off easily with a small crane, and to tie it down to the frame. The center pin on the underside fits into the fifth wheel, the beams on either end resting on the frame. Should there be an issue or problem, contact me via the designer link or via email to modelmechanic@gmail.com Be sure and follow me as a designer to get new release updates and easy access to my store.